1. Field of the Invention
The present invention relates to a microstructured body in which metal particles are arranged on a metal substrate having a surface with a structure of protrusions and depressions. In addition, the present invention also relates to a process for producing the microstructured body. Further, the present invention relates to a sensor device using the microstructured body, and a device for Raman spectrometry (Raman spectrometry device) using the microstructured body.
2. Description of the Related Art
Sensor devices and Raman spectrometry devices which utilize localized plasmon resonance occurring at a metal surface are known. Raman spectrometry is a technique of obtaining a spectrum (Raman spectrum) of Raman scattered light which is obtained by irradiating a material with monochromatic light and splitting the scattered light into wavelength components. Since the Raman-scattered light is weak, a Raman spectrometry technique called surface-enhanced Raman scattering (SERS), in which an electric field enhanced by localized plasmon resonance is used for amplifying the weak Raman-scattered light, is known.
The localized plasmon resonance is a phenomenon in which a strong electric field is produced around a nano-order protrusion of a metal surface by vibration of free electrons in the nano-order protrusion in resonance with the electric field of light which is incident on the metal surface, where the metal surface has a structure of nano-order protrusions and nano-order depressions.
A microstructured body in which a great number of particles are fixed to a substrate and at least the surfaces of the particles are formed of metal has been proposed as a sensor device or Raman spectrometry device which utilizes the localized plasmon resonance.
The patent document 1 discloses a technique for producing a Raman spectrometry device having a particle layer constituted by a great number of nonmetal/metal composite particles regularly arranged on a substrate. According to the technique disclosed in the patent document 1, the particle layer is formed by regularly arranging a great number of nonmetal particles such as silica particles on a nonmetal substrate such as a glass substrate, and the substrate on which the particle layer is formed is immersed in a solution containing metal and a polymer. Thereafter, the particle layer is dried, and baked at a temperature at which the polymer can be burned off.    Patent Document 1: Japanese Unexamined Patent Publication No. 2004-170334    Patent Document 2: Japanese Patent Application No. 2005-035564    Patent Document 3: Japanese Unexamined Patent Publication No. 2005-200677    Patent Document 4: Japanese Unexamined Patent Publication No. 2006-038506
According to the technique disclosed in the patent document 1, the nonmetal particles are not fixed to the substrate in the stage in which the particle layer is formed. Therefore, although the nonmetal particles are not fixed to the substrate, it is necessary to perform the operations of immersing the substrate (on which the particle layer is formed) in the solution containing the metal and the polymer, and then taking the substrate out of the solution. Since the nonmetal particles are not fixed to the substrate, it is very difficult to perform the above operations so that the nonmetal particles do not drop off the substrate and the regular arrangement of the nonmetal particles is maintained.
The present inventor has performed research of a process for easily obtaining a microstructured body having a nano-order metal structure of protrusions and depressions, and has previously invented two types of Raman spectrometry devices (1) and (2). The Raman spectrometry device (1) is produced by anodizing a portion of a metal body (for example, of aluminum) so as to transform the portion into a layer of a metal oxide (e.g., Al2O3) and removing the layer of the metal oxide from the metal body, so that the Raman spectrometry device (1) is realized by the unanodized portion of the metal body. The Raman spectrometry device (2) is produced by fixing a metal substance to a surface of the above unanodized portion of the metal body by evaporation or the like, where the constituent metal of the metal substance fixed to the surface of the unanodized portion is different from the constituent metal of the metal body. Since the Raman scattering intensity is effectively enhanced in the Raman spectrometry device (2), the Raman spectrometry device (2) is particularly desirable. The Raman spectrometry devices (1) and (2) have been disclosed in the patent document 2 by the applicant of the present application, and the patent document 2 has not yet been laid open at the time of filing of the present patent application.
Since the unanodized portion of the metal body is a metal body with a surface having a nano-order metal structure of protrusions and depressions (for example, as illustrated in FIG. 2(c) in the patent document 2), the Raman spectrometry device can be easily produced by merely performing the operations of anodizing a metal body, removing the anodized portion of the metal body, and fixing a different metal to the unanodized portion of the metal body by evaporation or the like. In addition, since the anodization can produce an approximately regular structure, a Raman spectrometry device having a highly regular metal structure of protrusions and depressions can be easily produced.
In the Raman spectrometry device (2), the form of the metal substance fixed to the depressions of the unanodized portion of the metal body is not specifically limited. The metal substance fixed to the unanodized portion of the metal body is a metal film in the example disclosed in the patent document 2. (See FIG. 1(b) of the patent document 2.) The patent document 2 does not teach a technique for fixing metal particles to the depressions of the unanodized portion of the metal body.
The patent document 3 discloses a technique for producing metal particles by obtaining as a base an unanodized portion of a metal body, selectively precipitating metal particles in depressions of a surface of the base by use of a plating technique, and removing the base.
The patent document 4 discloses a technique for fixing, by use of metal colloid, metal particles to a surface of a microstructured body constituted by an unanodized portion and an anodized portion, where the surface has protrusions and depressions.
However, it is difficult to selectively fix metal particles to the depressions of the surface by the plating technique or the technique using metal colloid.
The patent document 3 reports that the technique for precipitating the metal particles by the plating technique requires a contrivance such as use of a special additive. (See paragraph 0025 in the patent document 3.)
In case where metal particles are fixed by use of metal colloid, the dimensions of the metal particles are known. Therefore, it is necessary to arrange the depressions so as to have dimensions corresponding to the dimensions of the metal particles. The patent document 4 discloses that the anodized portion is not removed, the metal particles are fixed to the surface of the anodized portion, and pore widening processing is performed for enlarging micropores in the anodized portion (which correspond to the depressions) according to the dimensions of the metal particles in the colloid. (See paragraphs 0064 to 0070 in the patent document 4.)